1. Field of the Invention
The present invention relates to an inverter apparatus used for generating a three-phase AC voltage from a DC voltage, and more particularly, to an inverter apparatus suitable for battery vehicles, such as battery forklifts.
2. Description of the Related Art
One of requirements imposed on the inverter apparatus is that the inverter apparatus is designed to support various specifications required by users. One approach for satisfying such a requirement is to adopt module-based design. Japanese Laid-Open Patent Application No. JP-10-93220 discloses an inverter apparatus adopting module-based design, more specifically, an inverter apparatus in which component parts are optimally distributed to an insulated metal substrate and a printed circuit board. The inverter apparatus disclosed in this application incorporates power transistors on the insulated metal substrate, and control components and driving components on the printed circuit board.
In addition to the requirement described above, there are still other requirements imposed on the inverter apparatus adopting module-based design. Additional requirements imposed on the inverter apparatus are as follows:
(1) Improvement in Mechanical Strength, Including Vibration Resistance
The improvement in the mechanical strength, especially vibration resistance, is an important requirement imposed on the inverter apparatus mounted on battery vehicles. Components within the inverter apparatus mounted on the battery vehicle, such as cables, interconnections, circuit boards, electronic devices, and structural members, are subject to strong mechanical vibration. It is especially important that high current interconnections, such as bus bars, are designed to be resistant to mechanical vibration within the inverter apparatus.
(2) Optimization of Electrical Connections Between Insulated Metal Substrates and Printed Circuit Board
The electrical connections between the insulated metal substrates and the printed circuit board have influences on noise resistance. In addition, when the printed circuit board is provided with power supply and ground patterns that maintain a DC power supply voltage, and electrolytic capacitors connected therebetween, the optimization of the electrical connections between the insulated metal substrates and the printed circuit board have influences on the ratio of the printed circuit board occupied by the power supply and ground patterns. Forming wide power supply and/or ground patterns on the printed circuit board is effective for improving the noise resistance and for stabilizing the DC power supply voltage.
(3) Three-phase Symmetry in Circuit Layout
In order to achieve improved uniformity among U-phase, V-phase and W-phase currents, it is preferable that internal circuits within the inverter apparatus be arranged in symmetry with respect to the U-, V-, and W-phases.
(4) Prevention of Current Concentration on Specific Power Transistors
Power transistors may be connected in parallel within an inverter output stage when the inverter apparatus is designed to generate large output currents. In this case, it is preferable that uniform currents flow through the power transistors connected in parallel. Current concentration on specific power transistors is not preferable, since this may lead to damage to the specific power transistors.
These requirements are all desired to be satisfied under limitations on the volume of the inverter apparatus. For example, it is not preferable that the area occupied by the inverter output stage is increased in order to achieve improved uniformity of the currents though the power transistors. As far as the inventors know, no solution has been provided for the architecture of an inverter apparatus that satisfies these requirements on a practical level.